The present invention relates to a reciprocating-piston internal combustion engine with a piston which, in travelling to its upper dead center, sucks fresh gas via a controlled opening into the space beneath its base and, during the return stroke, compresses the gas and, via a connecting passage and another controlled opening, sends it to the cylinder of the internal combustion engine; and in the crankcase, the cross-section of the latter being of mainly circular form, with a connecting rod, which has unchanging dimensions in the direction of the axis of the crankshaft, including the ones of the crank eye, and which along its parallel front and back surfaces fits closely the mating even faces in the crankcase and thereby forms two separate spaces there, which by means of control devices form a connecting-rod pump.